As today's electronic devices become increasingly lighter, thinner, and smaller, there has been an attendant reduction in the thickness of the printed wiring boards that are installed inside these devices. A problem encountered with thin printed wiring boards is substrate warping. There are many causes behind substrate warping, such as warping caused by the manufacturing process, warping caused by temperature load, and warping caused by external load, and there is a particular need for warping caused by temperature load to be eliminated.
During reflow in the mounting of components, the substrate is put under a high-temperature environment of 200° C. or higher, and substrates made of composite materials consisting of copper foil, a base material, and a resist undergo warping due to the difference in the amounts of thermal expansion of the various constituent materials. Warping happens particularly often with portions to which the terminals, such as terminals of components and terminals directly connecting the substrate to the outside, are wired, because of the difference in the amounts of copper foil on the terminal face where the terminal is, and the terminal rear face on the opposite side. This warping causes improper mounting of the components, and therefore needs to be suppressed, but at present the most common approach is to employ a means such as providing perforations as disclosed in Patent Document 1 and elsewhere.
Other measures besides making perforations include a technique for suppressing warping by forming a dummy pattern corresponding to a warped state when a substrate warps, as disclosed in Patent Document 2.
Patent Document 1: Japanese Laid-Open Patent Application H9-8416
Patent Document 2: Japanese Laid-Open Patent Application 2000-151035